Cyclotron accelerator having the electrostatic field appearing across a nonlinear gap



Oct. 17, 1967 1 H THOMAS 3,348,089

GYCLOTRON ACCELERATOR HAVING THE ELECTROSTATlC FIELD APPEARING ACROSS A NONLINEAR GAP Filed July 29, 1963 2 Sheets-Sheet 1 FIG.I

INVENTOR. LLEWELLYN H. THOMAS ATTORNEY Oct. 17, 1967 L. H. THOMAS 3,348,089

4 CYCLOTRON ACCELERATOR HAVING THE ELECTROSTATIC FIELD LINEAR GAP APPEARING ACROSS A NON- Filed July 29, 1963 2 Sheets-Sheet 2 n t d tates Pate t 73cc 3,348 089 CYCLOTRON ACCELERATOR HAVING THE ELEC- TROSTATIC F HELD APPEARING ACROSS A N ON- LINEAR GAP Llewellyn H. Thomas, Leonia, N.J., assignor to Interna- V tional Business Machines'Corporation', New York, N.Y., a corporation'of New York Filed July 29, 1963, Ser. No. 298,022 Claims. (Cl. 313-62) This invention relates to an accelerator and, more particularly, to a cyclotron-type device 'for' accelerating particles.

The cyclotron is a well-known resonant accelerator and has been extensively described in the literature. Detailed descriptions of cyclotrons are found in texts entitled: Sourcebook on Atomic Energy, 'written by Samuel Glasstone, and published by D. Van NostrandCo, Inc, 1950 at pages 228244 and Introduction. to Nuclear Science, written by Alvin Glassner, published by D. Van N-ostr'and (30., Inc, 1961, at pages 1l2117. A conventional cyclotron is basically an evacuated chamber containing an arrangement of plates for the application'of an electrostatic field which, operating in conjuction with -a magnet, accelerates ions in circular orbits with expanding radii. The usual configuration of the plates is that of a relatively thin slice of a right circular cylinder that is bisected by a plane through its axis. A plate is often referred to as a dee because its cross-section is somewhat similar in shape to the character D. The magnet is located to provide magnetic field lines that are parallel to the axis of the cylinder. In operation, ions are injected into the center of the plate configuration and a high-frequency potential is applied to the plates. The electrostatic potential that appears across the slit or gap causes the ions to move from one side of the slit to the other. The magnetic field places the ions in orbits that are essentially circular and they return to re-cross the slit as the electrostatic field is reversed in polarity by a half alternation of the applied potential. As the ions are accelerated by successive crossings of the electrostatic field (gradient), their orbits increase in radii, and they approach the circumferential region of the plates. A deflector is arranged to remove the'acceler-ated ions to form the output of the cyclotron. The paths of the ions in a conventional cyclotron are described in detail in papers entitled The Paths of Ions in the Cyclotron, by Dr. Llewellyn H. Thomas, in The Physical Review, volume 54, Oct. 15, 1938, pages 580-598. The accelerated beams of ions generated by the cyclotron may be put to many uses including: medical treatment, production of radioactive isotopes, and nuclear experimentation.

The accelerator in the present invention provides an output ion beam which is many times more intense than that obtainable with a convention-a1 cyclotron. In the present invention the electrostatic field (gradient) does not appear across a linear gap between two semicircularshaped plates, but may assume many configurations. In the preferred embodiment, the field (gradient) occurs across a generally circular gap between a pair of circular plates and a pair of annular plates that surround the circular plates such that the gap appears at the circumference of the circular plates. In the conventional cyclotron, the loci of the equipotential electric field lines in the gap are essentially straight lines (with some distortion due to the compensatory broadening of the 'ends of the gap and the efiect of the supporting sides of the plate configuration). In the present invention, the locl of all equipotential field lines are essentially non-linear and preferably circular. In the invention, ions are accelerated in expanding orbits that intersect the gap at right angles. The centers of the orbits are in the gap when the orbits Cal 3,348,089 Patented Oct. 17, 1967 are small in diameter and they progress away from the center as the orbits expand'This motion of'thecenters accommodates the orbit size such that all orbits intersect the circular gap at essentially a right -angle.The radii of the orbits expands and contract, remaining at their maximum energy (largest orbital radius) for a relatively long period of time, providing an intense output and a relatively high' probability of internal high-energy collisions. The centers of the orbitsa'lso move circumferentially around the chamber toward a deflector which removes the intense-beam of accelerated ions as the cyclotron output.

Since the accelerator in the present invention provides such a high out-put intensity, its use is not limited to the uses of conventional cyclotrons. The present accelerator can be used to provide a source beam for other highenergy accelerators, such as linear accelerators. Furthermore, since the orbiting particles are extensively present at high-energy levels throughout the accelerator, other particles or gases (including plasmas) can be introduced directly into the accelerator as targets in the production of new particles which, in turn, are accelerated to useful energy levels. The accelerator in the present invention may also be used as the bombarding source in a thermo nuclear power system of the type described in a text entitled, Controlled Thermonuclear Reactions, by S. Glasstone and R. H. Lovberg, published by Van Nostrand in 1960 at page 65.

Accordingly, it is a primary object of the present invention to provide a resonant accelerator wherein the electric field lines are not parallel.

Another object is to provide a resonant accelerator having a non-linear gap between electrodes.

A further object is to provide a resonant accelerator wherein 'a gap between electrodes has a circular or other closed configuration.

A further object of the present invention is to provide a resonant'accelerator wherein the loci of all equipotential field lines are non-linear.

A further objectof the present invention is to provide a resonant accelerator wherein the loci of all equipotential field lines are circular.

Another object is to provide a resonant accelerator wherein the loci of all equipotential electric field lines are non-linear and where a gradient exists only in a relatively narrow region.

A further object is to provide a resonant accelerator wherein the loci of all equipotential electric field lines are circular and where a gradient exists only in a relative y narrow region.

A further object is to provide a resonant accelerator wherein an electrode structure supplies an electric field having non-linear equip-otential lines and is located in a magnetic field.

A further object is to provide a resonant accelerator having a magnetic field and two electrode structures which supplyxan electric field having non-linear equipotential lines in the magnetic field. i A further object is to provide a resonant accelerator having a magnetic field and an electrode structure surroundedby another electrode structure to supply an electric field having non-linear equipotential lines in the magnetic field.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

In the drawings:

FIGURE 1 is an isometric view of a preferred embodiment of the invention.

" V ciples of operation are considered.

3 FIGURE 2 is a cross-sectional view of the device showninFIGURE l. n

FIGURE 3 is a secondcr-oss-sectional view of the device showninFIGURE l.

Before considering the apparatus in dettil, the prin- Ionized particles,.including protons or deuterons, are

, introduced into the' chamber. Hydrogen'gas ca n al'so be Sintr'oducedsinto this chamber land, in this case, the hydrogen atoms (H diffuse throughout the chamber, and are struck by the energized'ionswhich have been accelerated by the applied RF field: These collisions produce additional ionized particles which are in turn accelerated.

.In the electric field-free region inside .the electrodes,

. I the ions are acted upon onlyby the substantially uniform magnetic field and travel in,-circular'orbits in a plane substantially normal to the magnetic field. After traversing V this circular path, which may be greater or less than 180 7 in ;a particular electrode, each ion returns to the gap be-" tween theelectrodes and comes again under the influence.

of the electric field where'its energy is again increased,

causing it to travel a path of larger radius. The radii of these Lar mor circles is given by the following expression:

where r is the robit radius, m is the mass of the accelerated particle (assumed non-relativistic), v is the velocity of this particle in the rth orbit, c is the velocity of light, e is the electronic charge of the orbiting particle, and B is the magnetic field strength. These orbits increase in size until the particles become out of phase with'the applied RF field and cease to be accelerated; this occurs 35 when'the particles have circular orbits which intersect 60"of the circumference of the inner electrode because the frequency of the applied RF voltage equals three times 'the cyclotron resonance frequency (to be definedjlater).

This orbit of'ma'ximum radius is shown in FIG..3. The

particles stay at this maximum orbit. for a time approximately equal to the time required to accelerate them to this maximum value, gradually changing phase while being accelerated at one crossing and decelerated at the next crossing until the phase is reversed. Then, the phase of the particles, with respect to the RF potential, is such that the particle is decelerated in crossing-the gap between the electrodes; The orbit radius then decreases to a mini mum value of zero. At nearly Zero energies (radii), the phase again changes. Then the particles are re-accelerated,

closely in phase, to their higher energies (larger radii) and again spend 'at that speed a time of the same order as the time of acceleration. This cycle is repetitive and to be the same as this ion revolution frequency, in the preferred embodiment, the third harmonic, or

V is chosen in'order' to provide the specified 'maximum' orbital energyand phase relationship. Ylith this frequency for the applied'RF voltage,'the subject device'is very'well suited for plasma bombardrnent within the chamber; A

mirror field for'confining plasma, which .is' introduced" V anywhere in the slot between the electrodes, .,can be 7 provided at radii of 7 (a is the etfective slot radius), said plasma being then bombarded by the accelerated particles. In this case, using the cyclotronresonance frequency for 'the frequencyof V V the applied RF'voi'tage, instead of the third harmonic, the particles would'be continuously accelerated until they collided with the wall and would pass through'the pla sma only once, reducing the probability of collision. Thus,

the third harmonic frequency is preferred when the device is used in this manner.

During the acceleration .phase, 'the particles acquire an increment 'of kinetic energy AT=eV ,V where v is the magnitude of the RF potential, each time that they cross the gap. Particles are moving in oppositedirections in f succeeding crossings, so the energy increments are cumulative. For N accelerations and an average potential difference V between the electrodes, the final energy is N Ve= /2mv This kinetic energy can also be written in terms of the magnetic field B and the final orbit;radius' The radius R applies to the enter 1t of the uniform mag- I netic field; the physical radius of the pole "faces must be larger by about /2 the gap length.

the orbits of all ionized particles within the chamber are The frequency of revolution in each circular path is given by I I v eB fi' 21rm with the previously-defined variables. This frequency is constant in the uniform magnetic field as long as the mass is constant, which is assumed to betthe case. Although the frequency of theapplied RF voltagecan be chosen Since the RF potential between electrodes variesrharmonically with time, particles can=be accelerated only during half of eachcycle; during the other half cycle they are decelerated. Essentially, all ions perform oscil-.

lations about the median plane and. they migrate in the phase in which they cross the gap. During each radio frequency cycle a loose bunch of ions start from a source,

and the center of the bunch follows the idealized exp'anding orbit described previously, The bunch includes those ions which are enclosed within an envelope inside the electrodes that is limited to a predetermined transverse ,7

spread about the median plane in; a' predetermined azimuthal sector. In each subsequent cycle another hunch is started and follows in the same track. The bunches V are discrete only for the first few revolutions; eventually they overlap and merge into an almost continuous radial distribution of all possible energies which is contained Within a limiting envelope of predetermined transverse dimension azimuthal region.

Conventional magnetic and electric focusing, such is described in the above-cited references, can be usedin the present invention. Electrical focusing is used when the time spent in the convergent field on entering is longer than that in a divergent field. This energy change effect is most pronounced in early accelerations when ion energy is'low and the change in velocity'is most significant. Also,

particles crossing the gap experience an electric field which changes during the time of transit. In that'portion of the RF cycle when the RF field is decreasing in'mag T nitude, the convergent force in entering the gap is larger in magnitude then the divergent force on leaving, so the net result is convergence. During the other quadrant of the accelerating half cycle the effects are reversed, resultmg in a tendency to divergence. As the particle energy is increased, this effect is lessened. i

The ions in the resonance beam start their acceleration near zero phase but experience phase shifts during acceleration. At all times, however, the ions of the resonant beam remain within the accelerating half cycle. For cyclotron resonance, the frequency of the applied electric field is equal to the ion revolution frequency or a harmonic of this frequency. Cyclotron resonance exists even without having electric field-free regions within the electrodes, as was assumed above. Although the paths are not semicircular, the resonance condition is still satisfied. A conventional technique can be used to control resonance in a cyclotron, that is, the magnetic field can be varied while the applied RF frequency is held constant. At the start of acceleration, the central field is higher than that specified by the applied frequency so the ions lead the voltage wave and the phase shifts toward 1r/2; at large radii the field is lower and the phase shifts in the opposite direction. When the ions reach their outer orbit and are to be deflected out of the chamber they should again be close to zero phase so the difference in radius of successive orbits is large enough for ions to be deflected behind a septum and enter the exit channel.

After acceleration to the designed maximum energy in the orbit radius R, the ions enter an exit channel which is defined by a septum (wall) of somewhat larger radius of curvature R'+A R' where as measured from the center of the electrodes. This channel can be placed anywhere in the periphery of the chamber. An electric field is maintained across the exit channel by a negative DC potential on insulated electrodes paralleling the septum. The difference in radius between the last two orbits is suflicient for a useful fraction of the beam to pass beyond the septum, where it is deflected outward in an open spiral path as an emergent beam. The present cyclotron provides a very intense output beam due to the fact that multiple particle orbits of the same radius can exist simultaneously within the chamber and these orbits precess throughout the 360 of the chamber. In this way, all particles which are accelerated exit through the deflector channel and contribute to the intensity of Y the emergent beam. The output beam intensity is further strengthened by the extensive ion presence in the vicinity of the deflector channel because the ions spend a considerable fraction of time in their maximum orbit.

Referring to FIG. 1, which illustrates the preferred embodiment of the invention, a conventional electromagnet is used to provide a nearly uniform magnetic field 12 between the flat faces of cylindrical pole pieces 1, 3 of large radius. A vacuum chamber 2, which contains a deflector apparatus 46, fits between the pole faces, and two hollow copper electrods 9, are mounted inside the vacuum chamber 2. The magnet poles 1, 3 are approximately 12 inches in diameter, and the vacuum chamber 2 is between 8 /2 and 9 inches in diameter. An RF generator 4 supplies the accelerating energy to the ions through coaxial leads 5 and the electrodes 9, 10. An exit port 6, into which is inserted an input lead 33 for a deflector 20, is connected to the outer electrode 10. Provision for obtaining a vacuum is provided by a pump 25 through a sealed port 7 and a vacuum seal 41. A voltage lead 8 through vacuum seal 41 and a gas port 36 through vacuum seal 45 provide the input requirements for an ion source 11. Water cooling for the electrodes is provided through the input ports 37 and vacuum seals 42. All individual components are discussed in greater detail with respect to FIGURES 2 and 3.

Referring to FIG. 2, which is a cross-sectional view of FIG. 1, the electromagnet has cylindrical tapered pole pieces 1 and 3 which are generally made from machined forgings, castings of soft iron, or of welded stacks of mild steel rolled plate. The pole pieces 1, 3 are tapered to keep the flux density approximately constant along the length of the pole. The diameter of the pole faces is approximately 12 inches and the magnetic gap is 3 inches. Design considerations allow variations of these measurements. The electrodes 9, 10 dimensions are based on the energy desired (or-bit sizes) and their choice controls the clearance between the electrodes 9, 10 and chamber surfaces. This clearance depends upon the designed maximum energy and the corresponding maximum electrode voltage, and to some extent on the smoothness of the pole faces. In this case, the inner electrode is 2 inches in radius and the radii of the outer annular electrode are 2%. and 8 inches. Controls can be p rovided for adjusting the mag nitude of the current and are used in tuning for peak beam current. Magnetic inhomogenities, such as the radial decrease required for focusing, the radial decrease at the periphery due to fringing, azimuthal variations of the field at all radii, and deviations of the magnetic median plane, can be corrected in the standard ways using corrective windings or shimming coils. Without disturbing synchronism, the magnetic field 12 on the central plane can be modified from where H is the magnetic field, h is the magnetic field at the geometrical center, K is a constant, and r is the radial distance along the central plane from the geometrical center. Many references are available for the design of a proper magnet, such as are found in Particle Accelerators, by M. Stanley Livingston and John P. Blewett, McGraw-Hill Book Company, 1961, at pages 236-283.

The chamber 2 which fits between the pole pieces 1, 3 of the electromagnet and which contains the electrodes 9, 10, the ion source 11 and the deflecting electrode 20, is vacuum tight and mechanically designed with adequate strength to resist distortion when under vacuum. The chamber is constructed of non-magnetic materials in order that no disturbance to the symmetrical magnetic field 12 is presented and is of high electrical conductivity to provide a low impedance for radio frequency currents. In addition, the chamber is equipped with a large number of scaled ports and apertures for inserting the many electrodes and controls. Generally, the chamber is a framework of thick walls 13 with many ports through the sides and with large circular apertures top and bottom filled by iron chamber lids 14 which are extensions of the magnet pole pieces 1 and 3.

The structural frame of the chamber 2 can be formed of rolled brass hoops or it can be welded structures of thick plates of non-magnetic stainless steel. In addition,

soldered or brazed assemblies of copper-alloy plates can.

be used as well as vacuum-tight bronze castings. In this latter case, all surfaces are machined, and many tapered screw holes are provided to bolt ports over the apertures and retain vacuum seals.

The soft iron chamber lids 14 rest on ledges 15 machimed in the chamber walls, with faces accurately parallel. The circular edge is sealed by a gasket joint 16 under pressure of a packing ring heldby a ring of bolts. Also,

Water-cooled copper sheet liners 17 on the inner faces of the lids 14 could be used to provide a high-conductivity surface for RF currents.

The ion source 11 is located on the floor of the chamber 2 against one pole face. A typical ion source can be used, such as a hooded-arc ion source or a hollow-anode ion source, both of which are discussed in numerous texts and articles, including the above-cited Livingston and Blewett reference at pages 159-163. In the preferred embodiment, the ion source 11 can be placed anywhere around the 360 of the slot 19 as the ions will disperse throughout the cyclotron. The exit port 21 of the ion source 11 does not project into the internal portion of the electrodes 9, in order that it not be severely damaged f byaccelerating particles. 7 a V An input voltage lead- 3 and an input gas lead ,36 for the ion source 1 1' is brought into the chamber wall. 13

- through a vacuum seal 45; i

The RF generator 4 is of conventional design, which,

in'fthe preferred embodiment, provides a controllable 1; radio frequency potential difference of approximately 10 kv. at a. frequency of approximately 90 me. betweenthe opposing faces of the electrodes 9 and 10. The frequency 'f is selected according to: .7 a

' neB v 21rm' I where n is a positive integer indicating the harmonic, e is the electroniccharge'of the particle to beaccelerated,

B is the magnetic field strength, andm is the mass of the particle to be accelerated (non-relativistic mass). The frequency of 90-mc. which is used in the preferred embodiment is the third harmonic (n=3) calculated for a;

proton. When a deuteron is to be accelerated, a third harmonic frequency of approximately 45 mo. is 'used.

The electrical system is mechanically'rigid to maintain chamberZ while the inner electrode 9 is held' securely by the insulators 23, Particles upon entering this gap. 19 I a stable frequency and has a high electrical efiiciency (high Q) to keep power requirements reasonable. The oscillator circuit 4 maintains suitable electrode voltage under all conditions of ion loading, recovers automatically from extreme load conditions such as gas discharges or sparks, and protects the power elements from damage.

The resonant circuit composed of the electrodes 9, 10 and their associated leads 5, is electrically equivalent to a pair of quarterwave coaxial transmission lines. The coaxialinput lines 5 are of large diameter /z inch) and are solidly constructed. Insulators '24 and a solid insulator 23, with their associated vacuum seals 39, are provided *to insure support of the inner electrode 9. These lines 5 are made of copper and are water-cooled by tubes (not shown) soldered to the inner surfaces 22 to obtain low resistance and high Q. The outer conductors of the coaxial input leads 5 provide shielding against radiative losses. The oscillator circuit 4 is capable of driving this cyclotron through varied conditions of sparking and discharge without the necessity of tuning or of manual resetting of overload relays. Operation is independent of minor frequency variations caused by ion loading or by vibrations or warping of the electrodes 9, 10. The circuit 4 provides an electric field to sweep ions and electrons out of the chamber rapidly enough to prevent cumulative ionization and is able to pull out of the blue-glow discharge condition (low voltage, high current discharge set up initially due to gas ionization) automatically. Rep

resentative circuits of this type are found in' an article by I. L. Kirchgessner, D. A..Barge, G. K. ONeil, G. H. Rees, I. Riedel, entitled The RF'System for the Princeton- 7 Pennsylvania Accelerator, published in the Transactions of the Professional Group on Nuclear Science of the Institute of Radio Engineers (now the Institute of Electrical and Electronics Engineers), vol. N8 9, No. 2, April 1962, atpages 11-18.

Generally, the vacuum chamber 2 is of large volume; and because of this factor and the presence of a large amount of gas evolution that occur in discharges,-a highspeed vacuum pump 25 is usedl'we'll-known oil-diffusion pumps of large diameter which have two or three pumping'stages and automatic fractionation of the oil are Well an input port 7 and an associated vacuum' seal 18.

FIG. 3 is a sectionaltop view'of the apparatus showing the deflector 46 and the maximum particle orbit 26, the bottom pole piece 3, the electrodes 9, 10 and the ion source 11. The ion source 11 and magnet were described above. The inner electrode 9 is a circular plate to which RF energy is applied. The outer electrode 10' is 'an annulus about this circular disc 9 such that there exists a 'radially constant gap 19 between theelectrodes 9, 10.

' I suited to this purpose; this pump 25 is connected through This outer electrode 10 is rigidly mounted to; the vacuum are accelerated' by the RF' voltage between the electrodes '9' and wand follow trajectories which are circles of in creasing radii; these trajectories are such tha'tthecentf' of each circle precesses about the entire gap 19.2The maximum orbit 26, where ais the effective (mean) slot 19 radius, is reached when this path makes 'a 60 angle with thein n er radius 27 of the annular outer electrode 10, as shown in the drawing,'when the RF'frequency equals the third harmonic of the cyclotronfrequencyw The minimum orbits occur when the" particles are out of phase with the RF voltage and are decelerated; the.

radius of this minin lum orbit is Zero.

" As described above, the resonant 1 provides an electric field to deflect the ,ions out of the chamber. The deflected beam traverses an opening spiral a as the ions cross the weakening magnetic field at the pole edges and pass out, of thechamber 2 through the port.6,- capped with a plate 34 which retains the vacuumthrouglu.

. a seal 44, but which does not stop the high energy particles. i Y

.External targets are placed beyond this plate The exit channel 30 is tapered slightly to accomodate the diverging beam 40. The septum' 28 defining the. exit slit 30 is located as close to the maximum energy orbit' as possible. The septum 28 is made of a thin solid-metal sheet (eg. tungsten). Since the septum may-be bombarded by a considerable fraction of the resonantfion j beam, it is cooled to preventdamage. The deflecting electrode 20 is mounted to the outside wall 31' of the annular electrode 10 through an electrical insulator 32.

The DC potential to the deflector 20 is provided by a .1 a

conventional high voltage rectifier unit at a therminal 29 and is applied on the input lead' 33 through a vacuum seal 43. The deflector unit can be located'anywhere on the periphery of the outer electrode 10 as all ions precess around the slot and eventually emerge'through the exit port 6.

Probe targets may be used to intercept the resonant beam although the preferred embodiment shows'anexit.

port 6 for removal of energized ions. The probe targets are mounted on stems with sliding vacuum seals at the chamber wall. Water cooling can be provided for these J targets. Beryllium-targets are particularly suited to the production of alarge number of neutrons by the reaction below: 7

A highenergy resonant accelerator has been shown and described. This accelerator has a design thatis radi cally different than any known eccelerator in that particles are simultaneouslyaccelerated in many different orbital paths. This feature enables an output beam to be produced which has significantly more energy then can be generated by a comparable single-path accelerator.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made ons spend-some time at their maximum orbit 26 before being slowed downd V A deflector 29 is attached to pull the resonant ion beam I i out'of its circular path'26 as the emergent beam which. i may be directed against an external target, if desired." I

therein Without departing from the spirit and scope of the invention.

What is claimed is: 1. A particle accelerator comprising, in combination: a vacuum chamber; means for applying a magnetic field across the chamber; and means for applying an alternating electric field whose equipotential contours are conic sections. 2. A particle accelerator comprising, in combination: a vacuum chamber; means for applying a magnetic field across the chamber; and means for applying an electric field having a configuration wherein the equipotential contours are non-linear and where a gradient exists only in a relatively narrow region within the chamber, said region having the shape of an annular ring. 3. A particle accelerator comprising, in combination: a vacuum chamber; means for applying a magnetic field across the chamber; and means for applying an electric field having a configuration wherein the equipotential contours are nonlinear and where a gradient exists only in a relatively narrow region within the chamber, said region describing a closed path. 4. A particle accelerator comprising, a combination: a vacuum chamber containing particles; means for applying an alternating electric field for accelerating said particles in closed orbits which periodically expand and collapse; magnetic field means for superimposing precessional motion upon said particles, shifting said particle orbits circumferentially around the center of said chamber. 5. An apparatus comprising, in combination: a vacuum chamber; means for applying a magnetic field across the chamber; and means comprising a plurality of concentric electrodes for applying an alternating electric field having a configuration wherein the equipotential contours are non-linear and where a gradient exists only in a relatively narrow region between the electrodes. 6. The apparatus described in claim 5, wherein the equipotential contours are conic sections,

7. A resonance accelerator comprising, in combination:

a vacuum chamber;

means for applying a magnetic field across the chamber;

a first electrode structure in the vacuum chamber comprising two plates that are substantially circular and substantially parallel, and a second electrode struc ture in the vacuum chamber comprising two annular plates that are concentrically located, each in the plane of and surrounding one of the plates in the first electrode structure.

8. A resonance accelerator comprising, in combination:

a vacuum chamber;

means for applying a magnetic field across the chamher;

a first electrode structure in the vacuum chamber comprising a plurality of plates that are in diiferent planes;

a second electrode structure in the vacuum chamber comprising a plurality of plates, each in the plane of one of the plates in the first electrode structure, where an edge of each plate in the second electrode structure is separated from the closest edge of the corresponding plate in the first electrode structure by a gap whose center line is non-linear.

9. The resonance accelerator. described in claim 8, wherein a plurality of plates in at least one electrode structure have the same shape and are in parallel planes.

10. The resonance accelerator described in claim 9, wherein the gap has a constant thickness and is in the shape of an annular ring.

References Cited UNITED STATES PATENTS 2,615,129 10/ 1952 McMillan 328-234 2,721,949 10/ 1955 Gund et al. 313-62 2,803,767 8/1957 Baldwin 31362 2,943,265 6/ 1960 Kaiser 31362 X 3,051,868 8/ 1962 Redhead. 3,197,661 7/1965 Sinclair 3 13-68 JAMES W. LAWRENCE, Primary Examiner.

DAVID I GALVTN, Examiner.

R. IUDD, Assistant Examiner, 

1. A PARTICLE ACCELERATOR COMPRISING, IN COMBINATION: A VACUUM CHAMBER; MEANS FOR APPLYING A MAGNETIC FIELD ACROSS THE CHAMBER; AND MEANS FOR APPLYING AN ALTERNATING ELECTRIC FIELD WHOSE EQUIPOTENTIAL CONTOURS ARE CONIC SECTIONS. 